I think you want to play on the Regular difficulty with mods because they already factor in balance issues like difficulty and assume people will use the normal difficulty. Since it really isn't "normal" or "easy" or "hard" anymore. Those are Vanilla Options, and a mod like this has overhauled too much to compare.
It is still a good question because sometimes you're expected to use the highest difficulty or it will be unbalanced. If that's the case it is usually stated somewhere visible. You should really just try it and see if it's too hard for you. The big difference with Lux Invicta is the almost shattered world, where large countries and empires do not exist anymore. The starts are far more balanced than Vanilla, and most of the ruler stats are randomized. So the technology and wealth of your county is the way to judge difficulty.
I'm assuming you are finding the mod too easy, but maybe you are finding it too hard and think it is the hard difficulty? Personally, I always found most starts challenging at normal difficulties. I have no idea what playing on hard is like, but I believe it only grants bonuses or takes something from you.
Ok I just asked Zijistark, team leader for HIP about difficulty levels in CKII Mods.
The short answer is that the difference in difficulty is arbitrary. "Not much."
"Hard & Very Hard are just flat AI buffs, aside from some better marriage planning at Very Hard that may or may not still be the case with latest patches but once was."
I hope I helped.
